A major difference between book publishing today and book publishing 25 years ago is the practical power of the author brand in marketing. Multi-book authors can not only build their own followings in ways that can be usefully exploited, they now have an unprecedented capability to help each other.
Of course, they can do that best if they're "organized" in some way. But both of the most obvious potential organizers who deal with many authors - the publishers and the agents - have commercial and structural impediments to being as helpful as they could be, or as authors need them to be, at either of the new needs: helping authors be better marketers of themselves or getting them to act in a coordinated way to help each other.